Tumor necrosis factor-α (TNF-α, also known as cachectin) is a mammalian protein capable of inducing a variety of effects on numerous cell types. TNF-α was initially characterized by its ability to cause lysis of tumor cells and is produced by activated cells such as mononuclear phagocytes, T-cells, B-cells, mast cells and NK cells. There are two forms of TNF-α, a type II membrane protein of relative molecular mass 26,000 (26 kD) and a soluble 17 kD form generated from the cell-bound protein by proteolytic cleavage. TNF-α is a principal mediator of the host response to gram-negative bacteria. Lipopolysaccharide (LPS, also called endotoxin), derived from the cell wall of gram-negative bacteria, is a potent stimulator of TNF-α synthesis. Because the deleterious effects which can result from an over-production or an unregulated-production of TNF-α are extremely serious, considerable efforts have been made to control or regulate the serum level of TNF-α. An important part in the effort to effectively control serum TNF-α levels is the understanding of the mechanism of TNF-α biosynthesis.
The mechanism by which TNF-α is secreted has not previously been elucidated. Kriegler et al. Cell, 53:45 (1988) conjectured that TNF-α “secretion” is due to the converting of the 26 kD membrane-bound molecule by a then unknown proteolytic enzyme or protease. Scuderi et. al., J. Immunology, 143:168 (1989), suggested that the release of TNF-α from human leukocyte cells is dependent on one or more serine proteases, e.g., a leukocyte elastase or trypsin. A serine protease inhibitor, p-toluenesulfonyl-L-arginine methyl ester, was found to suppress human leukocyte TNF-α release in a concentration-dependent manner. Scuderi et. al. suggested that an arginine methyl ester competes for the arginine-binding site in the enzyme's reactive center and thereby blocks hydrolysis. The lysine and phenylalanine analogs of the inhibitor reportedly failed to mimic the arginine methyl ester. However, it was never shown that this compound acted by inhibiting a protease that cleaves the 26 kD TNF. More recently, it has been reported that metalloprotease inhibitors block the release of TNF from THP-1 cells. See Mohler et al., Nature 370:218 (1994); Gearing et al., Nature, 370:555 (1994); and McGeehan et al., Nature, 370:568 (1994).
Most, but not all, proteases recognize a specific amino acid sequence. Some proteases primarily recognize residues located N-terminal of the cleaved bond, some recognize residues located C-terminal of the cleaved bond, and some proteases recognize residues on both sides of the cleaved bond. Metalloprotease enzymes utilize a bound metal ion, generally Zn2+, to catalyze the hydrolysis of the peptide bond. Metalloproteases are implicated in joint destruction (the matrix metalloproteases), blood pressure regulation (angiotensin converting enzyme), and regulation of peptide-hormone levels (neutral endopeptidase-24.11).